We have found that the platelet derived growth factor (PDGF) which is released from platelets in wounds is a potent chemoattractant for smooth muscle cells and for fibroblasts. These studies show that the chemotactic activity of PDGF is separate from its mitogenic activity. Addition of PDGF, collagen and fibronectin markedly stimulates connective tissue formation in models of wound healing. Various studies suggest that smooth muscle cells are brought to areas of injury by chemotaxis and the altered wound healing underlies fibrotic conditions. Transformed cells lose their ability to respond to normal wound hormones.